It was a bit easier to find ideas for cutie marks for these guys since most of them have a recognizable symbol…except Harley Quinn, she would have gotten her cutie mark before her meeting with the Joker so it doesn’t quite fit her current personality… If its too hard to see its a psychiatry chair.
Joker is a batpony (or partially, given the hair) but he’s missing the wings. If you wanna know why he’ll probably just give you various contradicting stories.
Harvey Dent is half batpony half pegasus, literally split down the middle.
Poison Ivy pretty much had to be an earth pony given their affinity to plants and the earth.
Harley… just became an earth pony. I didn’t want her to have magic or the ability to fly.